A shaped expanded graphite article is produced through the steps of: treating a graphite material such as natural graphite, kish graphite, pyrolytic graphite, etc., with a strong oxidant or the like such as concentrated sulfuric acid, concentrated nitric acid, hydrogen peroxide, etc., to thereby form a graphite interlayer compound; rapidly heating the graphite material (i.e., oxidized graphite material), which includes therein the graphite interlayer compound, at a high temperature of, e.g., 950 degrees C. or higher for 1-10 seconds, so as to generate pyrolysis gas whose gas pressure is used to expand a space between graphite layers and to form expanded graphite particles; and compression-forming or roll-forming the expanded graphite particles with or without using a binder. The shaped expanded graphite article produced in the above-described manner has various excellent properties, and is used as heat insulating materials, cushioning materials, and the like. An expanded graphite sheet obtained by forming the expanded graphite into a sheet shape using a roll, etc., is also widely used as gaskets, packings, sealings, and the like.
Recent years see a rapidly increasing need for general industrial equipment used at a high temperature such as gaskets used in automobiles or industrial packings used in oil plants and the like. The packings for general industries and the gaskets for automobiles are used in the air (i.e., in an oxygen-containing atmosphere) and at the same time used at a high temperature of 600 degrees C. or higher. Accordingly, it is required that a shaped expanded graphite article used for such packings and gaskets has a sheet form and suffers little oxidation loss.
According to a technique proposed in Japanese Patent Publication No. 57-15046 possessed by the present applicant, an expanded graphite sheet contains a boron component derived from a borate ester in order to obtain improved oxidation resistance. However, when this expanded graphite sheet was tried out as a gasket and a packing, there arose a problem that the boron component decomposed and scattered at a high temperature.
The present applicant has also proposed producing an oxidation-resistant expanded graphite sheet that contains phosphorus pentoxide and phosphate (in International Publication WO01/05703A1). The expanded graphite sheet proposed therein incurs no problem when used in the air at 600 degrees C. However, in a higher temperature range, e.g., at 650 degrees C. or higher, a rate of its oxidation loss is rapidly increased because a larger amount of carbon combines with oxygen contained in the air. Thus, there has been a problem that such an expanded graphite sheet can never be used at 750 degrees C. or higher.
An object of the present invention is to provide a shaped expanded graphite article that suffers little oxidation loss even at a high temperature in an oxygen-containing atmosphere, and also to provide a method for producing such a shaped expanded graphite article.